Twitter users now Tweet one billion times every two and a half days, CEO Dick Costolo said at a talk held at University of Michigan’s Gerald R. Ford School of Public Policy. Costolo also repeated a promise that users would be able to download the full history of their Tweets by the end of the year, which depends, he caveats, entirely on the engineering staff’s ability to work nonstop up until then. [Source: TechCrunch / University of Michigan]